A conventional screw-type material treating machine, as shown in the accompanying FIGS. 1 to 3, generally comprises an elongated barrel 2 surrounding the screws 1a and 1b and, in the case of machines having two screws, the barrel 2 is provided with two cylindrical bores having parallel axes, the distance between the latter being less than the diameter of the bores so as to form two intersecting lobes 3a and 3b containing the screws 1a and 1b which are meshed with each other. In the region of their intersection two lobes 3a and 3b define two longitudinal edges 4 parallel to the axes of the screws 1a and 1b.
The screws 1a and 1b are provided on their periphery with helical flights which drive the material introduced through the upstream end of the barrel 2 to the downstream end of the barrel which may be provided with an extrusion die or a simple outlet orifice in the case where the material does not have to be extruded.
In order to vary the treatment achieved in the course of the advance of the material through the barrel 2, the screws 1a and 1b are usually made up of successive zones having flights of different shapes or pitches or comprising special treating elements.
For example, there may be employed conveying sections having large pitches or sections having narrowed pitches or even shearing sections which, among other things, contribute to the mixing and dispersion of fillers or pigments in the material, and which generate self-heating of the material by a viscous dissipation of the mechanical power.
For this purpose, the screws 1a and 1b include shearing elements 6a and 6b (FIG. 2) which may be formed by unmeshed depression-creating elements such as, for example, counterflights or kneading discs.
In order to vary the work done by the shearing elements 6a and 6b, escape chambers 5a and 5b are formed by a cylindrical bore of larger diameter than the bore of the corresponding lobe 3a and 3b.
It is known that, in the air gap 7 (FIG. 2) formed between the bore of the lobe 3a and 3b and the corresponding shearing element 6a and 6b, a shearing field prevails which contributes to the mixing and dispersion of the fillers in the material and which generates self-heating of the material.
By modification of the position of a movable barrel relative to fixed screws, or of the position of screws axially movable relative to a fixed barrel, the length of the air gap and consequently the shear stress is controlled and, by these means so as, the degree of self-heating or thermomechanical degradation of the treated material or the homogeneity of the mixture.
In order to obtain effective shearing of the treated material, the shearing zone must be constant and the intersection edges of the two lobes continuous so as to force the material to pass through the air gaps.
In prior art installations, the bores of the escape chambers 5a and 5b open out in the region of the edges 4 so that the intersections of the bores of the escape chambers 5a and 5b and the bores of the contiguous lobes 3a and 3b form, in the plane of the edges 4, two broken lines 4a, as shown in FIGS. 2 and 3.
These broken lines 4a create transverse escape paths through which the material flows, so that the adjustment of the length of the air gaps in accordance with the treated material cannot be achieved to obtain the desired quality of the material.